Wire harnesses which are used in aircraft, ships and in-ground installations are typically encased in a protective shield which is formed from multi-strand carriers which are braided onto the wire harness. The purpose of the shield is to protect the wire harness against lightning strikes and EMI. The conventional approach to providing the aforesaid protection has been to provide maximum area coverage for the wire harness with the braid, typically 85 to 95%. This approach has been taken in part because of the perceived need to provide high frequency (above 50 MHz) EMI shielding for the wire harness. The resultant braid has been conventionally formed with a relatively large angle of strand carrier application onto the wire harness, i.e., typically about 60 degrees.
The weight of the shielding is a significant factor in the overall weight of the wiring components in a facility which requires such shielding. The use of the wire harness braided shielding has increased in recent years due primarily to two considerations, which are:
1) the use of electronic systems to replace mechanical devices, especially in aircraft flight controls; and
2) the use of composite materials which utilize graphite to replace metal structures in aircraft and other craft
We have determined that electronic equipment can be protected against high-frequency EMI by filtering the input and output wiring using light weight components, such as pin-filter connectors, and that such filtering is sufficient to protect against frequencies greater than 50 MHz in most applications. Thus, a braided shield providing high frequency protection is unnecessary, and may only add weight and stiffness to the wiring assembly. These weight and stiffness characteristics are not desirable, especially in aircraft applications.
Wire shielding is required to provide low frequency shielding for lighning protection and to preclude interference due to low frequency external electromagnetic fields. The lightning requirement stems from large transient voltages that result when lightning current flows in a resistive structure. The external field requirement arises from the fact that filter components for low frequencies are physically large and therefore impractical for applications where weight is critical, and furthermore, that such filters may interfere with proper functioning of the equipment. Therefore, wire harness shielding is needed to prevent interference from low-frequency sources.
It would be desirable to provide a wire harness shielding braid which is both flexible and light-weight, and yet provides the necessary shielding protection.